Specific biochemical markers for the two major cell types of the nervous system have been determined. Thses proteins are isoenzymes of the glycolytic enzyme enolase and are specifically Neuron Specific Enolase (NSE) and Non-Neuronal Enolase (NNE). NSE is present in all types of neurons and NNE is localized in glial cells within the nervous system. Studies are currently directed at elucidating the physiological significance of these two structurally, functionally, and immunologically distinct proteins, and their utilization as specific markers in probing the functional organization of brain. Recent immunocytochemical findings have shown that during neuronal maturation NSE has also been demonstrated in peripheral neuroendocrine cells of the APUD category. The very recent development of a more sensitive radioimmunoassay has made possible clinical studies where levels of NSE and NNE in serum and cerebrospinal fluid will be correlated with neuronal and glial pathologies respectively.